1. Field
The present invention relates generally to optical systems, and more specifically to a high-efficiency beam director for use in telescopes or laser projection systems.
2. Background
A beam director directs light between a beam, such as light emanating from a distant source, and an image point. When used to collect light from a distant source and focus the light on a focal surface, a beam director can be used as a telescope. When used in the other direction, for example to propagate light collected from a point source to a distant target, a beam director can be used in other ways, such as for laser communications, laser target designation, and projection of directed energy laser weapons. In most applications, it is desirable for a beam director to maintain the lowest possible loss of the energy during propagation, both within and outside the beam director. Lowest loss of energy corresponds to highest efficiency. For clarity, efficiency is defined here as the ratio of usable light energy out of an optical system to the light energy into said optical system. Usable light energy excludes light that exits the system and is lost because of diffusion, scattering, etc.
Telescopes collect light from a distant source and focus that light onto a focal surface for viewing. To collect the same amount of light, a less efficient telescope must generally be made larger than a more efficient telescope to compensate for lost light. Similarly, a beam director used to project light from a point source over a long distance (in the opposite direction as a telescope) should be as efficient as possible. Loss of light energy during propagation can mean that the optical system must be larger, or the laser light source more powerful, than would otherwise be necessary to achieve the same resultant light energy at the distant target. Thus, efficiency is a key characteristic for building compact beam director systems.
Many existing telescope systems, such as Ritchey-Cretien, Cassegrain, Maksutov, Gregorian, and Schmidt optical systems, as well as other catoptric or catadioptric variants, employ two mirrored surfaces, commonly referred to as the primary mirror and the secondary mirror. These systems are referred to herein as “two-mirror systems.” Two-mirror systems share several disadvantages. First, the two mirrors must be designed and formed to match each other. Specifically, the optical characteristics of the two conic surfaces (e.g., curvatures, conic constants, aperture diameters, relative positions, absolute orientation, and other optical characteristics) must be complementary. For example, in a classical Cassegrain telescope, the two mirror system may be composed of conic surfaces such as a large concave primary parabola and a small convex secondary hyperbola. Any change in optical characteristics of one mirror but not the other may cause a mismatch that will degrade the operation of the system. In addition, any irregularities in the primary or secondary mirrors, such as ordinarily incident to manufacturing, may cause additional loss of light energy. Finally, even primary and secondary mirrors that are designed and formed accurately must be precisely aligned with each other to achieve maximal performance. Any misalignment causes loss of light energy. Lastly a standard two-mirror system has a central obscuration in the center of its optical path. In a system that projects light from a laser light source to a distant target, this can result in the loss of light from the center of the laser light beam. Loss of light from the center of the laser light beam is very important, because the light emitted from many laser light sources is most intense at the center. In such a system, the portion of the laser light beam having the greatest light energy density is wasted.
There is therefore a need in the art for a highly compact, lightweight beam director having high efficiency and little or no loss of light by the central obscuration. There is further a need for such a system to be robust to misalignment.